Summer monsoon could bring dry lightning to Bay Area

A shifting weather pattern in California could bring dry lightning to the Bay Area, along with a risk of rain and thunderstorms in the Sierra Nevada.
The change could seriously increase the risk of fire danger in the region as hot weather has dried out grasses and other wildfire fuels, leaving them prone to ignition. 

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The risk of dry lightning is due to a ridge of high pressure currently building over the Rocky Mountains, which is forecast to pull subtropical moisture from the Gulf of California northward. 

Roger Gass, a meteorologist at the National Weather Service Bay Area office, said the current models show there will be high cloud cover over the marine layer in the region and a small chance of widespread wetting rain of more than a tenth of an inch as the monsoonal moisture arrives.

“This type of setup … could produce dry thunderstorms and that’s what we’re most concerned about,” Gass told SFGATE. 

Gass added that the chance of lightning is relatively low, around 10% in the Bay Area, but said it’s the “kind of scenario that if a lightning strike were to occur, it would likely start a fire.”

In 2020, a massive dry lightning storm swept through California, igniting hundreds of fires that burned millions of acres. Gass said this storm is not expected to be as intense as the one in 2020, but meteorologists are monitoring closely in case the forecast changes.

The monsoonal moisture is expected to arrive Saturday night and stick around though Tuesday in the Bay Area, Gass said. 

Farther east in the Sierra Nevada, the pattern change is expected to bring an increased risk of wet weather along with the potential for dry lightning. Currently, there’s a 20% to 25% chance of thunderstorms in the Sierra Nevada due to the monsoonal weather through next week.

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The region is already facing increased fire danger with a red flag warning issued for northeastern California for Friday due to gusty winds and low humidity levels.

“With this upcoming multi-day event, potential lightning starts from strikes outside of rain cores, flash flooding, and strong gusty outflow winds will all be possible,” the forecast discussion issued by the National Weather Service office in Reno said.

Tyler Salas, a meteorologist at the National Weather Service Reno office, said as moisture continues to be pulled in from the gulf, heavy rain may arrive later in the week. 

“Earlier in the week, it’s kind of a transitory period where we go from dry thunderstorms to kind of a mix of wet and dry,” he said. “Then we finally kind of transition to the wetter side as we keep getting moisture entering the area.”